Method of encapsulating hard disc drive and other electrical components

ABSTRACT

A method for injection molding a layer of phase change material around a surface of a plurality of identical motor components or hard disc drive components which includes providing a plurality of motor components or hard disc drive components; placing a motor component or hard disc drive component in a mold cavity of an injection molding machine having a controllable fill rate and a controllable injection pressure; closing said mold cavity; injecting a molten phase change material into said mold cavity at a fill rate and injection pressure; monitoring pressure in the mold cavity; controlling the fill rate of molten phase change material to obtain said motor component or hard disc drive component with the phase change material thereon, having a reproducible resonance spectrum; and repeating the above steps to produce a plurality of motor components or hard disc drive components each having a substantially uniform resonance spectrum.

BACKGROUND OF THE INVENTION

The present invention relates generally to hard disc drives, hard discdrive components and other electrical components having a more uniformand predictable, and in some cases modified, resonance spectrum.Particularly, it relates to the structure, construction and arrangementof hard disc drive components or other electrical components to obtain amore uniform and predictable and otherwise improved resonance spectrum.

Computers commonly use disc drives for memory storage purposes. Discdrives include a stack of one or more magnetic discs that rotate and areaccessed using a head or read-write transducer. Typically, a high speedmotor such as a spindle motor is used to rotate the discs. Voice coilmotors are typically used in actuator assemblies to move the heads overthe discs.

In many electrically motorized hard disc drive applications, significantmechanical vibration and acoustic noise is generated from both themechanical and magnetic sources. Mechanical sources include, but are notlimited to, things such as static and/or dynamic imbalance of therotating parts, bearing elasticity and imperfections, windage, and othermechanical means of creating fluctuating forces. In an electric motor,magnetic sources include such things as the magnetostriction fromcommutation of the current in the electric coils, magnetic forceimbalance from arrangement of the poles, slots and coils, and magneticforce imbalance due to eccentricity of the rotor and/or the stator. Thevibration from mechanical and magnetic sources usually has an adverseeffect on the performance of the motorized spindle. In hard disc driveapplications, motor vibration creates undesirable acoustic noise,angular speed variations and data-track mis-registration. It istherefore desirable to reduce the sources of vibration as much aspossible.

In hard disc drive applications, it is desirable to have a drive thathas a predictable system-wide resonance. The various components in ahard disc drive have their own unique resonance spectrum when the discdrive is in operation. The combination of these resonance spectrumsdefine the system wide resonance spectrum of the hard disc drive.Components such as voice coil motors and spindle motors havesubcomponents which also have their unique resonance spectrum. Thecombination of the resonance spectrums of the motor subcomponents definethe system wide resonance for the motor. Sometimes a particularfrequency of vibration in one part can couple with the resonatefrequency of another part creating a node of energy that is undesirable.As an example a motor bearing may have a defect frequency at 1250 hertzwhich may excite a resonate frequency of the motor bracket causing asystem wide vibration of the motor assembly. Therefore it is desirableto tune the motor so that points of excitation can be manipulated toavoid this excitation phenomena. It is also common that differentmanufacturers install the various components in the hard disc drive.These variations in system wide resonance must be accounted for in themanufacturing process. A large range of variance in system wideresonance, is a limiting factor in designing servo control logic todrive the heads over the data, in turn limiting the ability of the headsto track the repeatable runout of the media as it spins in hard discdrives.

There is a need for design features and manufacturing techniques thatserve to reduce the variance in system wide resonance and obtain a morepredictable and uniform system-wide resonance for a hard disc drive andhard disc drive components. Therefore, the present invention provides amethod to obtain predictable and uniform system wide resonance as muchas possible and to tune the frequency of resonance to enable reductionsin sympathetic system wide resonances, thereby leading to lowervibration and noise. The present invention also provides a method ofenclosing components of the hard disc drive to obtain a predictable anduniform system wide resonance and to reduce both mechanical and magneticvibration and noise.

Methods to enclose components of the hard disc drive with a syntheticresin have been suggested, but have not been used to obtain more uniformand predictable system-wide resonance. Prior art methods to mold asynthetic material enclosing hard disc drive components fail to producea predictable system wide resonance due to several factors. First, theplastic material that is used to enclose the components has variationsfrom lot to lot. In particular, the plastic may vary in viscosity by 60percent from lot to lot. The molecular weight of the polymer moleculesalso begins to vary as heat is applied, causing the polymer molecules tobecome smaller, which causes variations in the viscosity of the moltenpolymer and stiffness of the solidified polymer. Second, the polymersexhibit non-newtonian rheology and the density of the polymer inside amold cavity is not uniform. Third, in the past, it has not been possibleto control process variables to obtain a relatively uniform volume ofpolymer, orientation of the polymer as it enters the mold cavity, norensure a uniform rate of crystallization as the polymer solidifies.

One example of an overmolded stator and a method of manufacturing such astator is shown in U.S. Pat. No. 6,075,304 (Nakatsuka) (incorporatedherein by reference). Referring to FIGS. 6 and 7 of this patent, astator 11 is encapsulated with an overmold 12. The patent discloses thatthe injection speed of the polymer should be more than twice theinjection speed of a standard injection molding process. The patent alsodiscloses a molding tool capable of pressure dampening to reduce rapidpressure increase. However, this patent does not teach how to obtainpredictable and uniform resonance for a hard disc drive or a hard discdrive components.

An example of a spindle motor is shown in U.S. Pat. No. 5,694,268(Dunfield et al.) (incorporated herein by reference). Referring to FIGS.7 and 8 of this patent, a stator 200 of the spindle motor isencapsulated with an overmold 209. The overmolded stator containsopenings through which mounting pins 242 may be inserted for attachingthe stator 200 to a base. U.S. Pat. No. 5,672,972 (Viskochil)(incorporated herein by reference) also discloses a spindle motor havingan overmolded stator. One drawback with the overmold used in thesepatents is that it has a different coefficient of linear thermalexpansion (“CLTE”) than the corresponding metal parts to which it isattached. This patent also does not teach a method or structure forobtaining predictable and uniform resonance.

U.S. Pat. No. 5,806,169 (Trago) (incorporated herein by reference)discloses a method of fabricating an injection molded motor assembly.However, the motor disclosed in Trago is a step motor, not a high-speedspindle motor, and would not be used in applications such as hard discdrives. The patent does not disclose how to obtain uniform resonance.Thus, a need exists for an improved hard disc drive, hard disc drivecomponents and methods for making the same that overcome theaforementioned problems.

BRIEF SUMMARY OF THE INVENTION

A hard disc drive has been invented which overcomes many of theforegoing problems. In addition, unique spindle motor assemblies,actuator assemblies and other components of a hard disc drive have beeninvented, as well as methods for manufacturing components for hard discdrives and other electrical components. In one aspect, the invention isa method for injection molding a layer of phase change material around asurface of each of a plurality of identical hard disc drive componentswhich includes the steps of: providing a plurality of identical harddisc drive components; placing one of said plurality of identical harddisc drive components in a mold cavity of an injection molding machinehaving a controllable fill rate and a controllable injection pressure;closing said mold cavity; injecting a molten phase change material intosaid mold cavity at fill rates and injection pressures; monitoringpressure in the mold cavity; controlling the fill rate and/or injectionpressure of said molten phase change material to obtain said hard discdrive component with the phase change material thereon; and repeatingthe above steps to produce said plurality of components each having asubstantially uniform resonance spectrum.

In another aspect, the invention is a method of manufacturing hard discdrives having a reproducible resonance spectrum that includes the stepsof providing a plurality of identical hard disc drive component sets,wherein each of said sets consists of components that are used in asingle hard disc drive; placing and positioning one of said plurality ofhard disc drive component sets in a mold cavity of an injection moldingmachine; closing said mold cavity; monitoring the pressure inside themold cavity at an end-of-fill point; injecting a molten phase changematerial into said mold cavity to a pre-determined set point pressure;and repeating the steps above to produce a plurality of hard disc driveseach having a substantially uniform resonance spectrum.

In yet another aspect, the invention is a method of reducing sympatheticsystem wide resonances of components in a hard disc drive that includesthe steps of: providing a hard disc drive component; determining adesired resonance spectrum of said hard disc drive component; placingsaid hard disc drive component in a mold cavity of an injection moldingmachine having a controllable fill rate and a controllable injectionpressure; closing said mold cavity; injecting a molten phase changematerial into said mold cavity at a fill rate and an injection pressure;monitoring the pressure in the mold cavity; and controlling the fillrate of said molten phase change material and injection pressure toobtain said hard disc drive component with the phase change materialthereon, having said desired resonance spectrum.

The advantages of this invention are reduction in variance of resonancespectrums of hard disc drive and other electrical components. Thisreduction in variance allows hard disc drive manufacturers the abilityto better design hard disc drives. Resonance spectra of electrical andhard disc drive components may also be altered to reduce sympatheticsystem wide resonances. Other advantages of the invention will becomefurther apparent from the following detailed description of thepresently preferred embodiments, read in conjunction with theaccompanying drawings. The detailed description and drawings are merelyillustrative of the invention an do not limit the scope of theinvention, which is defined by the appended claims and equivalentsthereof.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a partial cross-sectional view of a hard disc drive of thepresent invention.

FIG. 2 is a partial perspective view of a voice coil motor of thepresent invention.

FIG. 3 is a partial perspective view of a hard disc drive with the voicecoil motor of FIG. 2.

FIG. 4 a is a perspective view of a stator.

FIG. 4 b is a perspective view of a stator substantially encapsulated ina monolithic body of phase change material of the present invention.

FIG. 5 is a cross-sectional view of an injection molding machine thatmay be used to practice the present invention.

FIGS. 6 a and 6 b are cross-sectional views in open and closed positionsof a mold cavity that could be used with the injection molding machineof FIG. 5.

FIG. 7 is a flowchart illustration of the preferred injection moldingprocess of the present invention.

FIG. 8 is a graph illustrating the relationship between viscosity andflow rate for preferred polymers used in practicing the presentinvention.

FIG. 9 a is a graph illustrating the pressure at the end-of-fill pointfor multiple cycles for an example injection molding process withoutcontrol.

FIG. 9 b is a graph illustrating the relationship between pressure andtime in the runner and mold cavity for an example injection moldingprocess without control.

FIG. 9 c is a graph illustrating the pressure at the end-of-fill pointfor multiple cycles for an example injection molding process of thepresent invention.

FIG. 9 d is a graph illustrating the relationship between pressure andtime in the runner and mold cavity for an example injection moldingprocess of the present invention.

FIG. 10 is a table illustrating the first order resonance frequency forencapsulated voice coil motors of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS AND PREFERRED EMBODIMENTS OF THEINVENTION

Referring to FIGS. 1 to 3, there is shown an embodiment of a product ofthe present invention comprising a hard disc drive system 10 having oneor more parts that have a layer of phase change material thereonresulting in a more predictable system-wide resonance for the hard discdrive system 10. The hard disc drive system 10 is a combination of thehard disc drive components that make up a hard disc drive. Inparticular, it includes a combination of the spindle motor, base andvoice coil motor of the hard disc drive.

Referring to FIG. 1, the major elements of the hard disc drive system 10of the present invention are shown, including hard disc 100, spindlemotor assembly 200, and an actuator assembly 300. These components areattached to a base portion 108 of a housing. The base plate 108 ispreferably made of stamped steel. A shell portion forms a cover 111, andin conjunction with the base portion 108, encloses the aforementioneddisc drive components.

The disc 100 has a centrally located aperture through which a hub 102extends. The hard disc 100 is rotatably supported on the hub 102, whichis an integral part of the rotor 210 of spindle motor assembly 200. Inthe preferred embodiment of the present invention and as depicted inFIG. 1, one concentrically aligned disc 100 is positioned on the hub102. The disc drive depicted is a single disc system; however, toincrease storage capability, multi-disc systems are foreseeable.

As depicted in FIG. 1, the hard disc 100 is preferably rotated by thespindle motor assembly 200. In addition to integral hub 102, the spindlemotor 200 includes a stator 204, a rotor 210, a shaft 206, and bearings208. The stator 204 has a plurality of poles 207 with wire windings 205.The wire windings 205 serve as conductors and induce or otherwise createa plurality of magnetic fields when electrical current is conductedthrough the conductors.

In the present embodiment, the integral hub 102 is fixedly mounted toshaft 206 forming the axis of rotation of the motor 202. The shaft 206is mounted to the base plate 108 by gluing or other conventionalmounting means. Bearings 208 are journalled about the shaft 206 andsupport rotor 210 comprised of the hub 102 and a permanent magnet 214positioned on a outer surface of the hub 102 facing the stator 204. Theinteraction of a magnetic field generated by the stator 204 with therotor permanent magnets 214 propels the rotor 210 to spin. The rotor210, having the hub 102 as an integral component, rotates the hard disc100. In the preferred embodiment shown in FIG. 1, there is also ahousing 215 that houses bearing supports 208 and shaft 206. The base 215is not essential to practice the invention and can be removed, andinstead the hub 102 can be used to house the bearing supports 208 andshaft 206. Other motor configurations that can be manufactured usingconcepts of the present invention are disclosed in U.S. Pat. No.6,300,695 issued Oct. 9, 2001, incorporated herein by reference.

The actuator assembly 300 has a voice coil motor 400, as illustrated inFIG. 2, that drives an actuator arm 320 (FIG. 3) to pivot and swing backand forth over the disc surface 500 to read and write data. The actuatorassembly arm 320 is attached to a shaft 306 at one end. The other end ofthe actuator arm has a head 330 that reads and writes data. The shaft306 is mounted to the base plate 108 by gluing or other conventionalmounting means. Bearings 308 are journalled about the shaft 306. Thebearing supports 308 and shaft 306 are housed in a metal housing 310.The metal housing 310 is preferably made of steel.

Referring to FIG. 1, a monolithic body 250 of phase change material isinjection molded onto the non-moving components of the hard disc drivesystem 10. Although the embodiment in FIG. 1 shows all of the non movingparts injection molded with a monolithic body of phase change material,one of ordinary skill in the art will understand that any combination ofparts may be unitized with a monolithic body of phase change material tohelp obtain a predictable system-wide harmonic resonance for theunitized parts.

As shown in FIG. 2, an actuator motor or voice coil motor 400 issubstantially encapsulated with a monolithic body 450 of phase changematerial to unitize the non-moving subcomponents of the actuator motor400. Substantially encapsulated means that the monolithic body surroundsenough surface area of a component so that it effectively alters theresonance spectrum of that component to a single resonance spectrum orto a desired resonance spectrum. A pole piece 414 is disposed beneaththe portion of actuator 300 which incorporates coil 416, and which incooperation with magnet 418 and pole piece 414, functions to driveactuator 300 about bearing 308 and pivot axis 422. The monolithic body450 unitizes the subcomponents of the actuator motor. The unitizedsubcomponents behave as a single component and have the same resonancespectrum and vibrational characteristics. FIG. 3 illustrates aperspective view of a hard disc drive showing the placement of actuator300 in a hard disc drive. The unitized actuator motor formed by themethod of injection molding of the present invention has a reproducibleresonance spectrum.

Illustrated in FIG. 4 a is a stator 204 having a plurality of poles 207with wire windings 205. As illustrated in FIG. 4 b, in anotherembodiment of the present invention, only the stator 204 issubstantially encapsulated with a monolithic body 450 of phase changematerial with the method of the present invention to obtain a statorassembly 216 having a reproducible resonance spectrum.

The phase change material used to make the body is preferably athermally conductive but non-electrically conductive plastic. Inaddition, the plastic preferably includes ceramic filler particles thatenhance the thermal conductivity of the plastic while improving the lossfactor or ability to damp vibration. A preferred form of plastic ispolyphenyl sulfide (PPS) sold under the trade name “Konduit” by LNP.Grade OTF-212 PPS is particularly preferred. Examples of other suitablethermoplastic resins include, but are not limited to, thermoplasticresins such as 6,6-polyamide, 6-polyamide, 4,6 polyamide,12,12-polyamide, and polyamides containing aromatic monomers,polybutylene terephthalate, aromatic polyesters, liquid crystalpolymers, polycyclohexane dimethylol terephthalate, copolyetheresters,polyphenylene sulfide, polyacrylics, polypropylene, polyethylene,polyacetals, polymethylpentene, polyetherimides, polycarbonate,polysulfone, polyethersulfone, polyphenyloxide, polystyrene, styrenecopolymer, mixterus and graft copolymers of styrene and rubber, andglass reinforced or impact modified versions of such resins. Blends ofthese resins such as polyphenylene oxide and polyamide blends, andpolycarbonate and polybutylene terephthalate, may also be used in theinvention.

The hard disc drive components of one embodiment of the presentinvention are insert molded with a monolithic body of a phase changematerial that unitizes the subcomponents of the hard disc drivecomponents. The hard disc drive system has a body of phase changematerial that unitizes some or all non-moving components of the harddisc drive.

The hard disc drive and its motor assemblies include one or more, andgenerally a plurality of solid parts to be used in the motor either nearor within the body, such as bearings and inserts. In addition, there aresolid parts that are near the body, such as a disc support member and ahard disc drive base. The preferred method of developing the hard discdrive comprises designing a phase change material to have a coefficientof linear thermal expansion such that the phase change materialcontracts and expands at approximately the same rate as the one or moresolid parts. For example, the preferred phase change material shouldhave a CLTE of between 70% and 130% of the CLTE of the core of thestator. The phase change material should preferably have a CLTE that isintermediate the maximum and minimum CLTE of the solid parts where thebody is in contact with different materials. Also, the CLTE's of thebody and solid part(s) should preferably match throughout thetemperature range of the motor during its operation. An advantage ofthis method is that a more accurate tolerance may be achieved betweenthe body and the solid parts because the CLTE of the body matches theCLTE of the solid parts more closely.

Most often the solid parts will be metal, and most frequently steel,copper and aluminum. The solid parts could also include ceramics. Inalmost all motors there will be metal bearings. Thus it is preferredthat the phase change material have a CLTE approximately the same asthat of the metal used to make the bearings.

Most thermoplastic materials have a relatively high CLTE. Somethermoplastic materials may have a CLTE at low temperatures that aresimilar to the CLTE of metal. However, at higher temperatures the CLTEdoes not match that of the metal. A preferred thermoplastic materialwill have a CLTE of less than 2×10⁻⁵ in/in° F., more preferably lessthan 1.5×10⁻⁵ in/in° F., throughout the expected operating temperatureof the motor, and preferably throughout the range of 0° F. to 250° F.Most preferably, the CLTE will be between about 0.8×10⁻⁵ in/in° F. andabout 1.2×10⁻⁵ in/in° F. throughout the range of 0° F. to 250° F. Whenthe measured CLTE of a material depends on the direction of measurement,thickness of the sample, or conditions of molding, the relevant CLTE forpurposes of defining the present invention is the CLTE of anencapsulated component in the direction in which the CLTE is lowest.Preferably, the CLTE in other directions is not more than 4 times thelowest value. The CLTE values are measured by a standard ASTM testmethod where the phase change material has the shape and form of themonolithic body that is overmolded on a component. The CLTE of commonsolid parts used in a motor are as follows:

23° C. 250° F. Steel 0.5 0.8 (× 10⁻⁵ in/in ° F.) Aluminum 0.8 1.4Ceramic 0.3 0.4

Of course, if the motor is designed with two or more different solids,such as steel and aluminum components, the CLTE of the phase changematerial would preferably be one that was intermediate the maximum CLTEand the minimum CLTE of the different solids, such as 0.65 in/in° F. atroom temperature and 1.1×10⁻⁵ in/in° F. at 250° F.

One preferred thermoplastic material, Konduit OTF212-11, was made into athermoplastic body and tested for its coefficient of linear thermalexpansion by a standard ASTM test method. It was found to have a CLTE inthe range of −30 to 60° C. of 1.09×10⁻⁵ in/in° F. in the X direction and1.26×10⁻⁵ in/in° F. in both the Y and Z directions, and a CLTE in therange of 100 to 240° C. of 1.28×10⁻⁵ in/in° F. in the X direction and3.16×10⁻⁵ in/in° F. in both the Y and Z directions. (Hence, the relevantCLTEs for purposes of defining the invention are 1.09×10⁻⁵ in/in° F. and1.28×10⁻⁵ in/in° F.) Another similar material, Konduit PDX-0988, wasfound to have a CLTE in the range of −30 to 30° C. of 1.1×10⁻⁵ in/in° F.in the X direction and 1.46×10⁻⁵ in/in° F. in both the Y and Zdirections, and a CLTE in the range of 100 to 240° C. of 1.16×10⁻⁵in/in° F. in the X direction and 3.4×10⁻⁵ in/in° F. in both the Y and Zdirections. By contrast, a PPS type polymer, (Fortron 4665) was likewisetested. While it had a low CLTE in the range of −30 to 30° C. (1.05×10⁻⁵in/in° F. in the X direction and 1.33×10⁻⁵ in/in° F. in both the Y and Zdirections), it had a much higher CLTE in the range of 100 to 240° C.(1.94×10⁻⁵ in/in° F. in the X direction and 4.17×10⁻⁵ in/in° F. in boththe Y and Z directions).

In addition to having a desirable CLTE, the preferred phase changematerial will also have a high thermal conductivity. A preferredthermoplastic material will have a thermal conductivity of at least 0.7watts/meter° K using ASTM test procedure F 433 and tested at roomtemperature (23° C.).

Referring to FIG. 5, an injection molding machine is used to manufacturea hard disc drive or hard disc drive components having a reproducibleresonance spectrum. The injection molding machine is similar to themachines used conventionally in thermoplastic injection moldingprocesses. A unique aspect of this invention is the method for injectionmolding a layer of phase change material onto the hard disc drive orhard disc drive components. The injection molding apparatus suitable foruse in the method provided by the present invention comprises aninjection cylinder 12 having a resin feeding screw 13 inside, a moldcavity 22, an runner 65, a stroke sensor 60 and pressure transducers P1,P2 and P3.

The molten material flows into the mold cavity 22 via runners 65. Gatesare placed at the end of the runner to control the flow of moltenmaterial into the mold cavity. Valve gate 50 opens and closes the runner65 to the cavity 22. Suitable valve gates are any valves known in theinjection molding art.

However, it is also possible to perform the method of the presentinvention without the use of a valve gate. In a process where no valvegates are used, the molten material is kept at a predetermined pressurein the mold cavity and is allowed to solidify. The mold cavity is openedand the part and the solidified material in the runner are ejected andthen separated. The use of a valve gate eliminates the need for theseparating step.

In a preferred embodiment, a hard disc drive component such as a voicecoil motor, spindle motor or stator is insert molded with a monolithicbody of phase change material to obtain a component with a reproducibleresonance spectrum. In the alternative, a hard disc drive with a base,spindle motor assembly and actuator assembly can be insert molded with amonolithic body of phase change material to obtain a reproducibleresonance spectrum. In one embodiment, as illustrated in FIG. 6 a andFIG. 6 b, a stator is placed into a mold cavity 22. The mold cavity isdesigned to hold the stator and form a predetermined shape. Retractablepins 76 hold the stator in place during the injection molding process.They are later retracted once the mold cavity is filled with phasechange material. The injection molding method begins with closing themold cavity as illustrated in FIG. 6 b and opening the valve gates 50.Molten material 55 fills cavity 22. A stroke sensor 60 measures the rateof plastic injection. A controller 70 correlates this rate, thecompressibility of the plastic and the size of the injection unit todetermine a quantity of plastic injected with time. A pressuretransducer P1 is associated with the beginning-of-fill point and isplaced near the gate 50 of the mold cavity 22. The beginning-of-fillpoint is the first portion of a mold cavity that is filled by moltenmaterial. Thus, the pressure transducer P1 is preferably placed withinthe first ten percent of the mold cavity to be filled by moltenmaterial. Another pressure transducer P2 is associated with theend-of-fill point in cavity 22. The end-of-fill point is the lastportion of a mold cavity that is filled by molten material. Thus, thepressure transducer P2 is preferably placed within the last ten percentof the mold cavity to be filled by molten material. Also a pressuretransducer P3 is placed in the runner 65 to monitor the runner pressure.The stroke sensor 60, as illustrated in FIG. 5, measures the fill rateof the molten phase change material.

Molten material enters through the gate and quickly fills up the entirecavity. The stroke sensor 60 and pressure transducers P1, P2, and P3transmit their respective readings to a controller 70, as illustrated inFIG. 5, which is preferably used in the method of the present invention.The controller 70 uses the pressure and stroke readings to determinewhether to increase or decrease injection pressure and fill rate toachieve a desired fill profile and pressure gradient. FIG. 9 dillustrates an example of a pressure profile at the various pressuretransducers for the present invention. Additionally the controller canbe used to close valve gate 50 and to stop the flow of molten materialinto the cavity 22. The controller reduces the flow of molten materialwhen the pressure at the end-of-fill point inside cavity 22 reaches aset point pressure. If valve gates are not utilized, the controllermaintains a constant injection pressure until the material in the runnerto and mold cavity have solidified. When the pressure at the end-of-fillpoint inside cavity 22 reaches the set point pressure, the moltenmaterial is allowed to cool and solidify. Although the embodimentdescribed above uses only one cavity, it is contemplated that multiplemold cavities maybe utilized to simultaneously carry out the method ofthe present invention.

FIG. 7 is a flowchart illustration of the injection molding process ofthe present invention. It will be understood that each step of theflowchart illustration can be implemented by computer programinstructions or can be done manually. These computer programinstructions may be loaded onto a computer or other programmable dataprocessing apparatus to produce a machine, such that the instructionswhich execute on the computer or other programmable data processingapparatus create means for implementing the functions specified in theflowchart step. These computer program instructions may also be storedin a computer-readable memory that can direct a computer or otherprogrammable data processing apparatus to function in a particularmanner, such that the instructions stored in the computer-readablememory produce an article of manufacture including instruction meanswhich implement the function specified in the flowchart step. Thecomputer program instructions may also be loaded onto a computer orother programmable data processing apparatus to cause a series ofoperational steps to be performed on the computer or other programmabledata processing apparatus to produce a computer implemented process suchthat the instructions which execute on the computer or otherprogrammable apparatus provide steps for implementing the functionsspecified in the flowchart step.

It will be understood that each step of the flowchart illustration canbe implemented by special purpose hardware-based computer systems whichperform the specified functions or steps, or combinations of specialpurpose hardware and computer instructions, or can be done manually.

An injection molding machine utilizing an injection molding process forthe present invention may have a single or multiple mold cavities. Theprocess begins with step 100 by positioning the hard disc drive and orits component(s) into the mold cavities. The cavity is then injectedwith molten material using a resin feeding screw at a predetermined fillrate in step 110. The fill rate is monitored and controlled to maintaina desired viscosity. The time it takes to fill the cavity to theset-point pressure depends on the size of the cavity and the injectionrate. In a preferred embodiment where a phase change material is moldedaround the surface of a voice coil motor, the injection rate is high.For example for overmolding a voice coil motor the injection rate isabout 25 cm³/sec at its maximum and it takes about 0.2 seconds to fillthe cavity to the set point pressure. In step 120, the pressure at theend-of-fill point inside the cavity is measured and compared to theset-point molding pressure. The set-point pressure is predeterminedbased on the size, shape and properties of the phase change material andthe materials of the hard disc drive components. For injection molding athermoplastic material on a voice coil motor for example, the set-pointpressure is preferably about 3,500 PSI. The process goes back to step110 if the pressure at the end-of-fill point inside the cavity is lessthan the set-point molding pressure. As the pressure inside the cavityapproaches and reaches the set-point pressure, the injection pressure,commonly referred to as packing pressure, is reduced in step 130.Because of pressure drops across the runner and gate into the cavity,the injection pressure is usually much higher than the cavity pressure.After all of the cavities are full at the set-point pressure, a constantpacking pressure is maintained and the molten material inside thecavities is allowed to cool and solidify in step 140. The injectionmolding process ends with step 150, when the components are ejected fromthe molding cavities.

By measuring the pressure at the end-of-fill point and holding thepressure inside the cavity at the set-point pressure, the phase changematerial will have more uniform density. Furthermore, the pressuregradient across the cavity is also more uniform and does not varysignificantly. The phase change materials used to form the monolithicbody are preferably plastics. Plastics are made of long polymer chainmolecules which cause them to behave in a non-newtonian manner. Asillustrated in FIG. 8, non-newtonian behavior with plastic materials ischaracterized by the fact that their viscosities change dramaticallywith shear rate. Changes in injection rate or gate size can impactviscosity and thus cavity fill rate and the cavity pressure gradient.Additionally, materials vary in viscosity from lot to lot whenmanufactured and can degrade with the presence of moisture or excesstemperature. Thus it is important to take advantage of this shearthinning to modify the rheology from lot to lot to a specific targetvalue. Consequently, when all the parameters, such as the fill rate andinjection pressure are set the same, parts can vary dramatically fromlot to lot. As explained in the background, due to the nature of longchain molecules and the polymerization process, there are inherentlylarge variations in the process even when machine conditions are heldconstant. However, variables such as pressure, temperature, flow rateand cooling rate do influence the properties of the phase changematerial which correlate to the finished part properties. By monitoringthese variables, part characteristics can be accurately and consistentlypredicted.

Each of the variables of the plastic injection molding process directlyor indirectly affect other variables which account for most variationsin the finished part. For instance, changing the barrel temperature onan injection molding machine would also affect the ability of thematerial to transmit pressure into the mold, and thus the plasticpressure is changed. As the material is heated, it becomes less viscousand thus the flow rate increases. Also, by increasing the temperaturethe cooling rate is affected, since the cooling rate is a function ofplastic temperature and mold temperature. Thus the injection moldedvariables are not well isolated or controlled.

Mold cavity pressure, however, has been found to be the most importantindicator of molded part dimensions and weight. Plastic pressure in themold cavity, however, cannot be expressed as a single figure for themold, but rather as a profile across the mold cavity. This profilebegins at the beginning-of-fill point and continues to the end-of-fillpoint. The flow restriction caused by the mold cavity and insertgeometry cause a pressure drop between the first and last areas to fillwith plastic. Thus the pressure is slightly different at differentpoints in the mold cavity. This gradient can cause non-uniformity of thephase change material across the mold cavity. Uniformity of theproperties of the phase change material is critical to obtaining areproducible resonance spectrum. Therefore it is important to ensurethat the pressure gradient inside the cavity is minimized and isreproducible for each shot.

It has been found that using pressure transducers at thebeginning-of-fill and end-of-fill points and measuring the injectionrate can provide a basis for intelligent decision making when coupledwith monitoring of the injection pressure and fill rate in the injectionmolding machine. With these four readings, one can determine thepressure gradient across the system and can alter other system variablesto obtain a target pressure profile across the mold cavity.

The pressure profile in the mold derived from transducers at thebeginning-of-fill and at the end-of-fill points provide the greatestdegree of insight into part quality. Because plastic is compressible atone half to three quarters of a percent per thousand PSI, this cavitypressure profile gives a measure of how the plastic is compressed acrossthe mold. After post-mold stabilization, which occurs sometime between 6hours to 6 days after the part is removed from the mold, parts achieveconstant density. Because material density varies across the mold priorto stabilization, due to the pressure profile, stabilized parts willchange size in varying amounts across the part in order to reach theirstable constant densities. If the pressure profile is kept uniform frompart to part, the density of the plastic is also quite uniform from partto part. This maintains the desired shape of the plastic afterstabilization and also retains its predicted spectrum of resonance afterstabilization for each part. Thus, in order to obtain a uniform pressureprofile, the pressure is preferably measured at the end-of-fill pointuntil it reaches a pre-determined set point pressure. At that point theflow rate is decreased and the molten material is held inside the cavityat the pre-determined set point pressure to obtain a target pressureprofile inside the cavity and allowed to cool and solidify.

The cavity pressure profile is influenced by not only the injectionpressure applied by the injection molding machine, but also by the filltime, temperature of the plastic and temperature of the mold. In apreferred embodiment, the fill time is also monitored and controlled tokeep a constant viscosity of the molten phase change material. Thepreferred fill time can be determined by comparing fill times of partsknown to be good during initial quality control studies on the mold.Cavity pressure profile with fill time held constant is a function ofthe apparent viscosity of the material, the melt temperature, and themold temperature. The temperature can be monitored and controlledthrough the use of thermocouples. Due to the non-newtoniancharacteristics of plastic, the fill time is preferably kept relativelylow, for example less than a second, with a high flow rate to decreasethe viscosity of the plastic and obtain a more uniform pressure profile.For phase change materials having ceramic filled particles, keeping theviscosity low also helps prevent agglomeration of the particles, whichcan affect the system wide resonance spectrum of the component.

FIGS. 9 a-9 d illustrate the difference between a process that does notuse control with the controlled process of the present invention. In anuncontrolled process the parameters are not controlled to ensure thatthe end-of-fill pressure reaches a predetermined set point pressure withevery shot. FIG. 9 a illustrates a graph showing the end-of-fillpressure for various shots in an uncontrolled process. Each peak andvalley point on the graph represents the end-of-fill pressure for asingle shot. As can be seen, in an uncontrolled process, the end-of-fillpressure varies considerably from shot to shot.

FIG. 9 c illustrates the end-of-fill pressure of a controlled process ofthe present invention. Each peak and valley point on the graphrepresents the end-of-fill pressure for a shot. As can be seen, in thecontrolled process of the present invention, the end-of-fill pressure isrelatively constant from shot to shot. The maintenance of thisuniformity of the end-of-fill pressure from shot to shot is asignificant contributing factor to the ability of obtaining areproducible resonance spectrum.

FIGS. 9 b and 9 d illustrate the pressure profiles of the injectionpressure 500, beginning-of-fill pressure 510 and end-of-fill pressure520 without control and with control respectively. As can be seen, theinjection pressure 500 is higher than both the beginning-of-fillpressure 510 and the end-of-fill pressure 520. Once the cavity is nearlyfilled with molten material, the injection pressure 500 is reduced andthe pressure at the beginning-of-fill begins to rise. Once the cavity isfull, the injection pressure is kept constant and the end-of-fillpressure rises to the predetermined set point pressure, in the case withcontrol. The part is then allowed to cool and solidify, which causes asignificant decrease in the end-of-fill pressure.

By injection molding a phase change material on one or a set of harddisc drive components in accordance with the method of the presentinvention, it has been found that one could obtain a reproducibleresonance spectrum for the molded components. All components have aresonance spectrum. The resonance spectrum of a component can bemeasured using the industry standard “ping” test. To perform the pingtest, the component placed in a clamp next to a microphone or it can beplaced in an acoustic chamber. The component is then hit with an impacthammer. A signal processor then measures the harmonic oscillations ofthe component. A component will have a resonance spectrum that will bedefined by peaks at different orders. The first order frequency for acomponent has the highest energy emission peak.

Shown in FIG. 10 is a table that shows the first order frequencyresonance of an encapsulated voice coil motor obtained through the pingtest. The data in the table is from parts molded in three differentruns. In one run of seven parts, the end-of-fill pressure was held inthe range of 4400-4499 psi. In a second run of fourteen parts, theend-of-fill pressure was controlled to be in the range of 4500-4700 psi.In the third run of four parts, the end-of-fill pressure was controlledto be in the range of 4701-4800 psi. Each part (specimen) was subjectedto the ping test and its first order harmonic resonance frequency wasdetermined. The results are recorded in Table I, along with the average,maximum, minimum and standard deviation (Σ) of the resonance frequencyof the first order harmonic of the parts produced in that run. Themedian first order frequency is the median value of all first orderfrequencies for a batch of identical components molded under acontrolled profile. One should understand that by identical components,it is meant that the components are all the same type of component andthat they are produced through the same manufacturing process from thesame manufacturer. It does not mean that the components are identicaldown to the microscopic level.

A reproducible resonance spectrum for the present invention is definedsuch that a batch of one hundred components made by the method of thepresent invention would have a standard deviation of first orderfrequency that is less than about 300 Hertz. Preferably the standarddeviation is less than about 100 Hertz, more preferably less than about50 Hertz, and most preferably about 30 Hertz or less. It should be notedthat in the data reported in Table I, the standard deviation is onlyabout 20 to 30 Hertz at three different end-of-fill pressures.

The standard deviation of first order resonance frequency is preferablyat least about twenty-five percent less, and more preferably at leastabout fifty percent less, than the standard deviation of first orderresonance frequency for a batch of components overmolded without themethod of the present invention. Conventional injection moldingprocesses control the injection pressure and either the injection timeor the stroke of the extrusion screw in the injection molding machine.For example in a conventional process, the injection pressure will beset and then the molten material is injected into the mold cavity for afixed time or a fixed stroke distance of the extrusion screw.

Such methods have yielded voice coil assemblies that have a standarddeviation resonance spectrum that is about 300 Hertz. With the method ofthe present invention it has been found that a standard deviation, for acomponent such as a voice coil motor, that is preferably less than aboutthirty Hertz may be obtained.

Another advantage of the present invention is that a resonance spectrumof a component may be altered to avoid sympathetic system wideresonances of components in a hard disc drive. It is also interesting tonote that the average resonance for the data in Table I getsprogressively higher with higher end-of-fill pressures. To modify aresonance spectrum of a component a suitable phase change material isselected and the component is overmolded with a layer of phase changematerial in accordance with the method of the present invention. Theproper injection pressure and fill rate necessary to obtain the desiredresonance spectrum are then determined. The resonance spectrum is thenevaluated. If points of sympathetic excitation are noted, the density ofthe encapsulated part is altered via changing cavity pressure to a newvalue. Once an acceptable structure is defined, the process settings aredetermined. The ability to control the pressure and fill rate, andmonitor other parameters of the injection molding process in accordancewith the method of the present invention, provide an ability toreproduce the desired resonance spectrum with every molding cycle.

With a predictable system wide resonance, hard disc drive manufacturerscan utilize predictable system wide resonance hard disc drive componentsto better design more compact hard disc drives that produce less noiseor vibration. Furthermore, the actuator position can be adjusted tofollow the various positions of the media data tracks induced byvibrations by using a servo control. By reducing the variability of thefrequency of these vibrations, manufacturers are able to control theactuator with more accuracy, thus leading to the ability to place moredata tracks closer together. This offers the benefit of more storage permedia disc.

It is contemplated that numerous modifications may be made to thecomponents and methods of the present invention without departing fromthe spirit and scope of the invention as defined in the claims. Forexample, the method of the present invention can be used for othermotors and components besides hard disc drive components. Motors used inthe automotive industry such as windshield wiper motors, integralstarter/alternators, drive motors for hybrid electric vehicles,appliance motors for clothes washers and dish washer and components ofsuch motors can be encapsulated with a phase change material to reducevibrational noise and obtain more reproducible resonance spectrums.Accordingly, while the present invention has been described herein inrelation to several embodiments, the foregoing disclosure is notintended or to be construed to limit the present invention or otherwiseto exclude any such other embodiments, arrangements, variations, ormodifications and equivalent arrangements. Rather, the present inventionis limited only by the claims appended hereto and the equivalentsthereof.

1. A method of reducing sympathetic resonances of a component in a harddisc drive comprising: a) providing a hard disc drive component; b)determining a desired resonance spectrum of frequencies to avoid forsaid hard disc drive component; c) placing said hard disc drivecomponent in a mold cavity of an injection molding machine having acontrollable fill rate and a controllable injection pressure; d) closingsaid mold cavity; e) injecting a molten phase change material into saidmold cavity; f) monitoring and controlling the pressure in the moldcavity; and g) monitoring and controlling one or more of the fill rateof said molten phase change material and injection pressure to obtainsaid hard disc drive component with the phase change material thereon,having said desired resonance spectrum.
 2. The method of claim 1 whereinsaid desired resonance spectrum is achieved by tuning the fill rate andpressure to a predetermined set-point fill rate and a predeterminedset-point pressure.
 3. The method of claim 1 wherein the step ofcontrolling the pressure in the mold cavity is accomplished by openingand closing one or more valve gates associated with said cavity.
 4. Amethod of producing a motor component with a desired resonance spectrumcomprising: a) providing a motor component; b) determining a desiredresonance spectrum; c) placing said motor component in a mold cavity; d)closing said mold cavity; e) injecting a molten phase change materialinto said mold cavity; f) monitoring and controlling the pressure in themold cavity; and g) monitoring and controlling one or more of a fillrate and an injection pressure of said molten phase change material toobtain said motor component with the phase change material thereon,having said desired resonance spectrum.
 5. The method of claim 4 whereinthe step of controlling the pressure in the mold cavity is accomplishedby opening and closing one or more valve gates associated with saidcavity.
 6. A method of producing an electrical device with asubcomponent having a desired resonance spectrum comprising: a)providing a subcomponent of said electrical device; b) determining adesired resonance spectrum of that subcomponent; c) placing saidsubcomponent in a mold cavity; d) closing said mold cavity; e) injectinga molten phase change material into said mold cavity; f) monitoring andcontrolling the pressure in the mold cavity; g) monitoring andcontrolling one or more of a fill rate and an injection pressure of saidmolten phase change material to obtain said subcomponent with the phasechange material thereon, having said desired resonance spectrum; and h)assembling said electrical device using said subcomponent.
 7. The methodof claim 6 wherein the step of controlling the pressure in the moldcavity is accomplished by opening and closing one or more valve gatesassociated with said cavity.